1. Technical Field
The technical field is embedded capacitors in printed wiring boards (PWB). More particularly, the technical field includes embedded capacitors in printed wiring boards made from thick film dielectrics and electrodes.
2. Technical Background of the Invention
The practice of embedding high capacitance density capacitors in printed wiring boards allows for reduced circuit size and improved circuit performance. Capacitors are typically embedded in panels that are stacked and connected by interconnection circuitry; the stack of panels forming a multilayer printed wiring board. The stacked panels can be generally referred to as “innerlayer panels.”
Passive circuit components embedded in printed wiring boards formed by fired-on-foil technology are known. “Separately fired-on-foil” capacitors are formed by depositing and drying at least one thick-film dielectric layer onto a metallic foil substrate, followed by depositing and drying a thick-film electrode material over the thick-film capacitor dielectric layer and subsequently firing the capacitor structure under copper thick-film firing conditions. U.S. Patent Application Publication Nos. U.S. 2004/0099999 A1 and U.S. 2004/023361 A1 (cofired divisional) to Borland disclose such a process.
After firing, the resulting article may be laminated to a prepreg dielectric layer, and the metallic foil may be etched to form the electrodes of the capacitor and any associated circuitry to form an inner layer panel containing thick-film capacitors. The inner layer panel may then be laminated and interconnected to other inner layer panels to form a multilayer printed wiring board.
The thick-film dielectric material should have a high dielectric constant (K) after firing. A high K thick-film dielectric paste suitable for screen printing may be formed by mixing a high dielectric constant powder (the “functional phase”) with a glass powder and dispersing the mixture into a thick-film screen-printing vehicle. The glass may be vitreous or crystalline, depending on its composition.
During firing of the thick-film dielectric material, the glass component of the dielectric material softens and flows before the peak firing temperature is reached. It coalesces and encapsulates the functional phase during the hold at peak temperature forming the fired-on-foil capacitor structure. The glass may subsequently crystallize to precipitate any desired phases.
Copper is a preferred material for forming electrodes. A thick-film copper electrode paste suitable for screen printing may be formed by mixing copper powder with a small amount of glass powder and dispersing the mixture into a thick-film screen printing vehicle. However, the large temperature coefficient of expansion (TCE) difference between the thick-film copper and the thick-film capacitor dielectric, and shrinkage differences during firing often lead to tensile stress in the dielectric just outside the periphery of the electrode. The tensile stresses may result in cracking in the dielectric around the periphery of the electrode as shown in FIG. 1A and FIG. 1B. In extreme circumstances, the cracks can extend all the way down to the copper foil. Such cracking is undesirable, as it may affect long-term reliability of the capacitor. Alternative capacitor structure designs that eliminate the conditions that lead to such cracking would be advantageous.
The present inventors have provided novel method(s) of forming electrodes and inner layers, embedding thick-film fired-on-foil capacitors, and forming printing wiring boards which avoid this cracking in the dielectric. Additionally, the present inventors have developed the electrodes, inner layers, capacitors and printed wiring boards formed by these methods.